Two new studies of UK wildlife provide alarming evidence that many animal and plant species in Britain are in sharp decline due to human activity.

The authors of one report claim their findings support the hypothesis the world could be in the midst of a sixth mass extinction.

Falls in numbers of butterfly, bird and plant species could be due to habitat destruction and climate change.

Details of the studies are outlined in the latest issue of Science magazine.

In one study, researchers analysed data from six surveys of UK butterfly, bird and plant species produced over the last 40 years.

Markers of change

The argument amongst some naysayers is that... insects are nowhere near as vulnerable to extinction as plants, birds and mammals. The results show this isn't true

Prof John Lawton, Nerc chief executive

They found the majority of butterfly species, a total of 71%, had declined over 20 years.

The researchers found 54% of British bird species fell over 20 years and 28% of native plant species decreased over 40 years.

"There are simply no data sets that approach this detail and scale in the world," co-author Dr Jeremy Thomas, of the Natural Environment Research Council (Nerc), told journalists at a news conference in London.

While there is abundant data supporting declines in birds, until now there has been no comparable data for species loss in insects. Yet insects comprise 54% of the world's described species, with birds making up only 0.6%.

The large blue butterfly became extinct in Britain in 1979

Craig Hilton-Taylor, a conservation biologist at the IUCN-the world conservation union, told BBC News Online: "When (butterflies) go, plants that are dependent on them for pollination tend to decline and so on."

Professor John Lawton, chief executive of Nerc told BBC News Online: "The argument amongst some naysayers is that biologists exaggerate the threat because insects are nowhere near as vulnerable to extinction as plants, birds and mammals.

"The results show this isn't true."

Global situation

If this situation is reflected elsewhere in the globe, say the authors, it strengthens the view that the world is heading for the sixth mass extinction.

Other researchers have proposed the sixth mass extinction theory on the basis that global extinction rates are roughly two orders of magnitude greater than normal "background" rates of extinction observed in the past.

Dr Jeremy Thomas holds a selection of UK butterflies

The last of these events occurred 65 million years ago and resulted in the extinction of the dinosaurs.

"For any scientist to extrapolate from this to a world insect fauna is a huge jump. But it is the only firm evidence we've got at the moment," said Dr Thomas.

Lord May of Oxford, president of the Royal Society, the UK's academy of science, said: "These are dismaying trends. If this pattern holds more generally then estimates of global extinction rates - which are mainly based on birds and mammals - could err on the optimistic side."

Homeless species

Habitat loss through changes in land use by humans is one of the major factors effecting the decline in animal species, researchers believe.

In the case of butterflies, this is likely to be the result of the ploughing up of heathlands or woodlands. Pollution often then degraded the remaining, fragmented habitats, Dr Thomas added.

Tree sparrow populations have dramatically collapsed

Another study, published in the same issue of Science, indicates the deposition of nitrogen in British grasslands through intensive use of fertilisers in agriculture and fossil fuel combustion is leading to a reduction in plant species.

One of the major challenges facing conservationists hoping to reverse this decline is finding enough land to provide habitats for species.

"Because these habitats are scattered across the country, we also need to make sure there is enough linkage to make sure that the species can move between them," said Mr Hilton-Taylor.

In Dr Thomas' study, more than 20,000 volunteers submitted about 15 million records of species during the six surveys.